RU2363020C2 - Method and device to produce calibration filter for electromagnetic data - Google Patents

Abstract

FIELD: instrument making.

SUBSTANCE: invention relates to sea bottom sounding data processing. The proposed method comprises generating electromagnetic radiation for electromagnetic field components to be measured by the receiver. Note here that the radiation source is located close by the receiver so that the incident signal dominates over ascending signal reflected from geologic horizon above the receiver. Such measurements can be used in analysing in frequency bands to isolate the signal incident component and to design the receiver calibration filter. In subsequent processing, electromagnetic data incident component is removed. The proposed method is repeated for each receiver used in electromagnetic sounding.

EFFECT: new method and device for producing electromagnetic data calibration filter.

24 cl, 3 dwg

Description

FIELD OF THE INVENTION

The present invention relates to methods for generating a calibration filter for electromagnetic data and methods for processing electromagnetic data using such a formed filter. The present invention can be used, for example, in the formation of a calibration filter with a view to its use in electromagnetic profiling of the seabed.

State of the art

The technology of electromagnetic seabed profiling (EM-SBL) is a new tool for studying hydrocarbons, based on electromagnetic data, and is disclosed in the article by Eidesmo et al. (2002) “Profiling of the seabed, a new method for direct identification of remote hydrocarbon-containing sediments in deep-sea areas” , The Leading Edge, 20, No. 3, 144-152, and Ellingsrud et al. (2002) “Detection of Remote Hydrocarbonate Deposits Using Seabed Profiling: Results of Work in the Coastal Zone of Angola”, First Break, 21 No. 10, 972- 982. EM-SBL is a special application of controlled source electromagnetic sounding (CSEM). For many years, NWEM sounding has been successfully used to study ocean basins and active spreading centers. SBL is the first CSEM application for the remote and direct detection of hydrocarbons in the marine environment. The first two successful SBL studies published were conducted in the coastal zone of West Africa (the above mentioned Eidesmo et al. And Ellingsrud et al.) And in the coastal zone of central Norway, Resten et al., (2003) “Terrain profile studies in the Ormen gas field Lange, "EAGE, 65 ^th Ann. Internal Mtg., Eur. Assoc. Geosc. Eng., Extended Abstracts, P058. Both studies were conducted in a deep-sea environment (at a depth of more than 1000 meters).

The method uses a horizontal electric dipole source (HED), which emits a low-frequency electromagnetic signal into the seabed and down into the underlying sediment. Electromagnetic energy decays rapidly in conductive deep deposits due to pore space filled with water. In rocks with high resistance, such as sandstones filled with hydrocarbonates, and at critical angles of incidence, the energy propagates along the layers and is weakened to a lesser extent. Energy is refracted back to the seabed and emitted by electromagnetic receivers located there. When the receiver-source distance (i.e. offset) is approximately 2-5 times greater than the depth of the tank, the refracted energy from the high-resistance layers will exceed the directly propagating energy. The release of spreading along the layers and refracted energy is the basis of EM-SBL.

The capacity of the reservoir filled with bicarbonates should be at least 50 m in order to ensure the efficient passage of energy along formations with high resistance, and the thickness of the water should ideally be more than 500 m to prevent the imposition of satellite waves from the border with air.

The electromagnetic energy that is generated by the source spreads in all directions, and electromagnetic energy rapidly decreases in conductive underwater sediments. The distance over which energy can penetrate deep rocks is determined mainly by the power and frequency of the excited signal, as well as the conductivity of the underlying formations. Higher frequencies lead to a greater attenuation of energy and, consequently, to a smaller depth of penetration. Therefore, the frequencies used in EM-SBL are very low, typically 0.25 Hz. The dielectric constant can be neglected due to very low frequencies, the magnetic permeability is assumed to be equal to the permeability in vacuum, i.e. corresponding to non-magnetic deep rocks. Despite the extremely low frequencies used in EM-SBL, the fact that the technology is based on the use of electric current or scattering rather than the propagation of electromagnetic waves means that range and resolution are not limited to theoretical calculations of wavelengths.

In numerical terms, a hydrocarbon-filled reservoir usually has a resistance of several tens of ohm-meters or more, while the resistance of higher and lower deposits is usually less than a few ohm-meters. The propagation speed depends on the environment. In seawater, the speed is approximately equal to 1700 m / s (at a frequency of 1 Hz and a resistance of 0.3 Ohm-m), whereas usually the speed of propagation of a magnetic field in underwater sediments filled with water is about 3200 m / s (at the same frequency and resistance 1 ohm-m). An electromagnetic field in hydrocarbon-filled layers with high resistance propagates at a speed of about 22,000 m / s (at a resistance of 50 Ohm-m and a frequency of 1 Hz). The depth of electromagnetic penetration for these three cases is approximately 275 m, 500 m and 3600 m, respectively.

Electromagnetic receivers can be placed separately at the bottom of the sea, each receiver measures two horizontal and orthogonal components of both electric and magnetic fields. The HED source consists of two electrodes spaced approximately 200 m apart and in electrical contact with seawater. The source transmits a continuous and periodically changing current signal with a fundamental frequency in the range of 0.05-10 Hz. The double amplitude of the speaker varies from zero to several hundred amperes. The height of the source relative to the seabed should be significantly less than the depth of penetration of electromagnetic energy in seawater to ensure good transmission of the excited signal to deep deposits, for example, 50-100 m. There are several ways to place receivers on the seabed. Typically, receivers are in a straight line. In studies, several such lines can be used, and the lines can have any orientation with respect to each other, for example, a certain number of lines can intersect.

The environment and equipment for recording EM-SBL data is illustrated in FIG. Research ship 1 tows an electromagnetic source 2 along and perpendicular to the lines of receivers 3, both linear (transverse magnetic) and broadband (transverse electric) energy can be recorded by receivers. The receivers on the seabed 4 produce a continuous recording of signals during the towing of the source at a speed of 1-2 knots. Sampling of EM-SBL data from a source is performed with a high density, usually with an interval of 0.04 s, with coarser quantization of the data from the receivers. Typically, the distance between the receivers is 500-2000 m. Therefore, standard processing and interpretation of the obtained data should be performed in the region of the common plane of the receivers, and not in the region of the common plane of the sources.

EM-SBL data is recorded as time series and then processed using windowed Fourier analysis of discrete series (see, for example, Jacobsen and Lyons (2003) Sliding DFT, IEEE Signal Proc. Mag., 20, No. 2, 74- 80) at the transmitted frequency, i.e. at the fundamental frequency or its harmonic component. After processing, the data can be reproduced in the form of graphs of the dependence of amplitude on offset (MVO) or phase on offset (PVO).

The combination of electrical and magnetic measurements can be used to decompose electromagnetic data into incident and ascending waves. This operation is known as the separation of the electromagnetic wave field, or the decomposition into ascending and falling components. In particular, separation of the electromagnetic field at the seabed level can be used to recognize and subsequently isolate or attenuate the incident air waves in the EM-SBL data. In addition, when the electromagnetic wave field is expanded, incident magnetotelluric waves (MT) are excluded. Usually there are differences in the effects of interaction and differences in the transient characteristics of the electrical and magnetic receiving installations. Therefore, measurements must be properly calibrated to correctly combine electrical and magnetic measurements into reliable vector representations.

Disclosure of invention

In a first aspect, the invention provides a method described in paragraph 1.

The following embodiments of the invention are characterized in other claims.

Thus, it seems possible to create a technology that will improve the determination of the optimal calibration filter for electromagnetic data. The technology is independent of the structure of the Earth, and no information about the Earth is required. The technology requires only measurements of electric and magnetic fields at a minimum source-receiver distance. Such a calibration filter can be used in the processing of electromagnetic data to separate the ascending and falling components in the received data and remove the incident satellite waves and MT.

Brief Description of the Drawings

For a better understanding of the present invention, as well as its practical implementation, preferred embodiments of the invention will be considered by way of example with reference to the accompanying drawings, wherein

figure 1 illustrates the environment and equipment for obtaining EM-SBL data;

FIG. 2 is a flowchart illustrating a method for manufacturing a calibration filter in accordance with an embodiment of the invention; FIG.

figure 3 is a schematic block diagram of a device for implementing the method illustrated in figure 2.

The implementation of the invention

The considered embodiment of the invention allows to create a technology for forming a calibration filter for processing EM-SBL data. It is assumed that electrical receivers have good interaction with the field and a frequency-independent calibration filter is determined for magnetic receivers. The application of this technology is equivalent in the opposite case, i.e. under the assumption that magnetic receivers have good field interaction and a frequency-independent calibration filter is determined for electrical receivers. Without loss of generality, the wave decomposition of an electric field is considered (more preferably than a magnetic field). The present technology uses a direct signal, i.e. A signal that travels directly from an electromagnetic source to electromagnetic receivers. It is advisable to place the source above or directly above the receivers. The technology can be fully automated and managed by existing data.

Next, we will consider the technology in which the receivers are located on the seabed, although it should be understood that this arrangement is purely approximate. The receivers can be located in any way relative to the electromagnetic source, but the source and receivers should be sufficiently close to each other so that the intensity of the rising field is negligible compared to the intensity of the incident field.

EM-SBL zero-offset data is a good approximation to purely incident waves. In real conditions, this requires the use of electromagnetic receivers with a large dynamic range, for example, 32-bit analog-to-digital converters.

связана с зарегистрированным скалярным электрическим полем, измеренным в х и у направлениях, E_x,y(ω), и зарегистрированным скалярным магнитным полем H_x,y(ω):In the frequency domain, the ascending component of the scalar electric field on the seabed

is associated with a registered scalar electric field measured in x and y directions, E _{x, y} (ω), and a registered scalar magnetic field H _{x, y} (ω):

and

,

,

where A (ω) is the decomposition filter, and ω is the angular frequency. The corresponding falling components are:

The vertical component of the scalar electric field E _z (ω) approaches zero directly below the source in a standard survey to obtain electromagnetic data. Corresponding expressions for the magnetic field can be obtained by applying the Maxwell equations. For vertically incident waves, A (ω) is determined by the expression

,

,

where µ is the magnetic constant, ε is the dielectric constant, and σ is the electrical conductivity. When the technology described here is applied to EM-SBL data, it can be assumed that the magnetic permeability corresponds to the permeability in free space, i.e. non-magnetic materials. The dielectric constant can be considered negligible due to the use of low frequencies that are used in EM-SBL. The electrical conductivity of seawater can be measured experimentally, although such measurements are not necessary at the location of each receiver. The electrical conductivity of shallow sediment on the seabed is also a measurable parameter, but this measurement is difficult to implement, so it is preferable to avoid the requirements for the value of this parameter.

, необходимо выполнить калибровку составляющих электрического и магнитного полей. Применение частотно-зависимого калибровочного фильтра В(ω) к магнитному полю дает следующее выражение для восходящего электрического поля:To satisfy the conditions of the above expression for

, it is necessary to calibrate the components of the electric and magnetic fields. Application of a frequency-dependent calibration filter B (ω) to a magnetic field gives the following expression for an ascending electric field:

and

A frequency-dependent calibration filter is obtained by eliminating the upstream direct signal.

According to a first embodiment of the invention, a frequency-dependent calibration filter B (ω) is obtained by applying the discrete Fourier transform to the localized time window of the electric and magnetic time series when the electromagnetic source is located above and / or in close proximity to the electromagnetic receivers. In this localized time window, the incident direct signal will largely dominate the recorded direct energy; ascending energy from deep horizons will be negligible. When the distance between the source and the receiver approaches the minimum, incident waves dominate in measurements on the seabed. The maximum distance between the source and receiver at which this technology can be implemented is approximately 1000 m.

и

являются соответственно представительными выборками измерений падающих электрических и магнитных сигналов в частотной области. В результате этого будут получены два выражения для калибровочного фильтра:In this embodiment, a known decomposition filter A (ω) with any of the parameters corresponding to the environment above or below the seabed is used to determine B (ω). This is achieved by equating equations 1 and 2 to zero, where

and

are respectively representative samples of measurements of incident electrical and magnetic signals in the frequency domain. As a result of this, two expressions for the calibration filter will be obtained:

and

When decomposition is performed above the seabed, then in accordance with the above description, it is necessary to know the electrical conductivity of sea water. This technology works well if the reflection coefficient from the interface between the sea water and the seabed is less than one third.

When decomposition is performed below the seabed, knowledge of the electrical conductivity of shallow sediments at the bottom of the sea is required. It is optimal to perform decomposition directly below the surface of the seabed. This is due to the fact that the reflection coefficients between layers in sediments are usually very small, as a result, the incident signal from the source in the zero offset region dominates the upward signals from deep deposits.

, сформированный из «истинного» калибровочного фильтра В(ω) и неизвестного фильтра разложения А(ω). Это достигается приравниванием выражений (1) и (2) нулю и переименованием

в эффективный калибровочный фильтр. В результате этого будут получены два выражения для эффективного калибровочного фильтра:According to a second embodiment of the present invention, an effective calibration filter can be determined

formed from a “true” calibration filter B (ω) and an unknown decomposition filter A (ω). This is achieved by equating expressions (1) and (2) to zero and renaming

into an effective calibration filter. As a result of this, two expressions will be obtained for an effective calibration filter:

and

Stable solutions to these equations are, for example:

and

where * denotes a complex conjugate operation and λ is a generalized stabilizing constant.

Calibration filters can be defined for several localized time windows, in which there will be a significant predominance of the incident direct signal in the recorded energy. Different solutions can be combined by linear (addition) or non-linear combinations.

An effective calibration filter can also be expressed in the time domain as:

and

,

,

where e (t) and h (t) are electrical and magnetic time series with offsets close to zero, -1 in the superscript indicates the inverse filter, and * indicates the convolution. Computationally, it is preferable to work in the frequency domain, although time-domain representation is included here for the sake of completeness.

неявно соответствует применению разложения непосредственно ниже поверхности морского дна, что в соответствии с приведенным выше описанием является оптимальным местом для выполнения разложения. Хотя эта технология и работает при условии, что падающая волна имеет большую интенсивность, чем восходящая волна, но более эффективным является тот случай, когда интенсивность восходящей волны составляет менее одной трети интенсивности падающей волны (т.е. коэффициент отражения между слоями в осадках меньше одной трети).The use of an effective calibration filter

implicitly corresponds to the use of decomposition directly below the surface of the seabed, which, in accordance with the above description, is the best place to perform decomposition. Although this technology works under the condition that the incident wave has a higher intensity than the rising wave, the case when the intensity of the rising wave is less than one third of the incident wave intensity (i.e., the reflection coefficient between the layers in the sediments is less than one thirds).

In a first embodiment of the present invention with a known decomposition filter A (ω), the electromagnetic parameters directly above or below the seabed should be known.

включает А(ω), и поэтому не требуется знание электромагнитных параметров морского дна.In a second embodiment of the present invention

includes A (ω), and therefore knowledge of the electromagnetic parameters of the seabed is not required.

A calibration filter obtained using either of the two options above can be applied to data recorded by receivers during the EM-SBL data acquisition process. With adequate calibration of the electromagnetic receivers, the incident waves can subsequently be removed by determining the shape of the rising waves.

The methods described in the above embodiments of the invention can be outlined by the flowchart of FIG. 2. At step 20, electromagnetic receivers are installed on the seabed, and at step 21, the source is located at zero or close to it offset above the first receiver. When the source emits energy (step 22), this receiver measures the electrical and magnetic components of the signal in the frequency domain (step 24). Under real conditions, the localized time series windows in the data obtained in step 25 undergo a discrete Fourier transform in step 26 to obtain electrical and magnetic signal components in the frequency domain.

The right branch of figure 2 shows the method implemented in the first embodiment. The known decomposition filter A (ω) is applied to the data in step 29.

The “true” calibration filter B (ω) is then generated in step 30. This process is repeated to generate calibration filters for each receiver (step 31).

The left branch of figure 2 shows the method implemented in the second embodiment. An effective calibration filter is determined in accordance with the above equations 3 or 4 (step 27). This process is repeated to refine the calibration filter for each receiver (step 28).

At a later stage or simultaneously with calibration measurements, an EM-SBL survey is performed (step 23), then the recorded data is processed using a calibration filter as part of the further data processing process (step 32), which consists in removing the incident components from the source in the signal and receiving MVO - or PVO characteristics as described above.

The data processing methods described above can be implemented by a program controlling a computer for implementing this technology. The program may be recorded on a storage medium, for example, hard or floppy disks, CD and DVD playable discs or flash memory. The program can also be transmitted over a computer network, for example, the Internet or a group of computers connected together in a local network.

The schematic diagram of FIG. 3 illustrates a central processing unit (CPU) 13 connected to a read-only memory (ROM) 10 and random access memory (RAM) 12. The CPU receives data 14 from receivers via an input / output unit 15. The CPU then determines a calibration filter 16 in accordance with the commands received from the memory block 11 with the recorded program, which can be part of ROM 10. The program itself or any input and / or output data can be received or transmitted through the data network 18, which can be, for example, The Internet. The same system or a separate system can be used to adjust EM-SBL data in order to remove incident signals to obtain modified EM-SBL data 17 for further processing.

For the specialist, the obvious possibility of numerous modifications in accordance with the above description without going beyond the scope of the invention defined by the formula.

Claims (24)

1. A method of forming a calibration filter for electromagnetic data, comprising the following operations:
generation by a source of electromagnetic radiation;
measuring the electric and magnetic components of the electromagnetic field generated by the specified source, in two orthogonal directions, at least one receiver, the source being located at a distance from at least one receiver close enough so that the intensity of the incident electromagnetic field is significantly higher than the intensity ascending electromagnetic field, to enable the formation of a calibration filter; and forming, on the basis of said measurement, a calibration filter for at least one receiver for using said filter in processing electromagnetic data. 2. The method according to claim 1, characterized in that the intensity of the ascending electromagnetic field is less than one third of the intensity of the incident electromagnetic field. 3. The method according to claim 1, characterized in that the source is placed at least over one receiver during the measurement process. 4. The method according to claim 3, characterized in that the source is placed directly on at least one receiver during the measurement. 5. The method according to claim 1, characterized in that at least one receiver is placed on the seabed. 6. The method according to claim 1, characterized in that the electromagnetic radiation generated by the source is transmitted to the seabed. 7. The method according to claim 1, characterized in that at least one receiver is used for electromagnetic profiling of the seabed (EM-SBL). 8. The method according to claim 1, characterized in that the calibration filter is formed for the magnetic receiver in at least one receiver. 9. The method according to claim 1, characterized in that the calibration filter is formed for the electrical receiver in at least one of the specified receiver. 10. The method according to claim 1, characterized in that the calibration filter is formed on the basis of analysis in the frequency domain of the measurement data made at least at one receiver. 11. The method according to any one of the preceding paragraphs, characterized in that the formed calibration filter is an effective calibration filter

obtained using the equation

Where

and

- measurements taken at least on one receiver of electrical and magnetic signals in the frequency domain in the orthogonal x and y directions, respectively, related by the expression

12. The method according to any one of claims 1 to 10, characterized in that the formed calibration filter is an effective calibration filter

obtained using the equation

Where

and

- measurements taken at least on one receiver of electrical and magnetic signals in the frequency domain in orthogonal x and y directions, respectively, related by the expression

13. A method of forming a calibration filter, characterized in that the calibration filter is a combination of various effective calibration filters obtained by the method described in paragraphs 11 and 12. 14. The method according to any one of claims 1 to 10, characterized in that the formed calibration filter is an effective calibration filter

obtained using the equation

Where

and

- measurements taken at least on one receiver of electrical and magnetic signals in the time domain in orthogonal x and y directions, respectively. 15. The method according to any one of claims 1 to 10, characterized in that the formed calibration filter is an effective calibration filter

obtained using the equation

Where

and

- measurements taken at least on one receiver of electrical and magnetic signals in the time domain in orthogonal x and y directions, respectively. 16. A method of forming a calibration filter, characterized in that the calibration filter is a combination of various effective calibration filters obtained by the method described in paragraphs 14 and 15. 17. The method according to any one of claims 1 to 10, characterized in that the generated calibration filter is a true calibration filter B (ω) obtained using the equation

Where

and

- measurements of electrical and magnetic signals in the frequency domain, performed at least on one receiver in orthogonal x and y directions, respectively, and A (ω) is a known decomposition filter, which is determined by the expression

18. The method according to any one of claims 1 to 10, characterized in that the formed calibration filter is a true calibration filter

obtained using the equation

Where

and

- measurements of electrical and magnetic signals in the frequency domain, performed at least on one receiver in orthogonal x and y directions, respectively, and A (ω) is a known decomposition filter defined by the expression

19. A method of forming a calibration filter, characterized in that the calibration filter is a combination of various true calibration filters obtained by the method described in paragraphs 17 and 18. 20. The method according to claim 1, characterized in that the calibration filter is formed using conversion in the frequency domain of the measured data in a localized window in which the intensity of the incident field exceeds the intensity of the ascending field. 21. A device for forming a calibration filter for electromagnetic data, containing a source of electromagnetic radiation; at least one receiver configured to measure the electric and magnetic components of the electromagnetic field generated by the specified source in two orthogonal directions, the source being located at a distance from at least one receiver close enough so that the intensity of the incident electromagnetic field is substantially exceeded the intensity of the ascending electromagnetic field to enable the formation of a calibration filter; and means for forming, on the basis of said measurement, a calibration filter for at least one receiver for using said filter in processing electromagnetic data. 22. A method for processing electromagnetic data using a calibration filter, comprising the following operations:
the formation of the calibration filter in accordance with the method described in any one of claims 1 to 20;
applying a calibration filter to electromagnetic data subsequently recorded by at least one receiver;
and the separation of the falling and ascending components of the electromagnetic data. 23. The method according to item 22, wherein the incident component of the electromagnetic data is removed. 24. The method according to item 22 or 23, characterized in that the data is EM-SBL data, and at least one receiver is placed on the seabed.

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